A. Technical Field
The present invention relates to integrated micromechanical systems and, more particularly, to systems, devices, and methods of reducing stiction failures in Micro Electro Mechanical System (MEMS) sensors.
B. Background of the Invention
Stiction phenomena are well-known problems in MEMS type devices with movable parts. Stiction effects typically occur between two surfaces when an external force deflects a movable part in a manner so as to cause its surface to come in physical contact and adhere to the surface of an adjacent stationary part.
Sensor type MEMS devices are particularly vulnerable to stiction, which may occur intermittently during regular use of the device or during the manufacturing process. Stiction adversely affects device performance and may be caused by a variety of forces including capillary forces caused by the presence of moisture and van der Waals forces caused by surface contamination, such as polishing residuals that may fluctuate depending on surface preparation processes.
For example, in a MEMS accelerometer sensor, an external disturbance such as a mechanical shock may deflect a suspended proof-mass in a manner so as to cause a portion of its surface to contact and adhere to an adjacent wafer substrate surface. When the total adhesion force between the two surfaces is higher than the mechanical restoring force inherent to the proof-mass, stiction occurs and temporarily immobilizes the proof-mass and prevents it from recovering its original position even after the external disturbance ceases to act on the sensor. This prevents the accelerometer from producing an accurate acceleration signal, until the stiction force is overcome, for example, by a sufficiently large counteracting force.
Since stiction causes the proof-mass to adhere to the substrate, the two parts are no longer separated from each other, blocking the movement of the proof mass and, in some cases, also causing a short circuit event that destroys the electric field between the two surfaces. Therefore, the sensor can no longer measure capacitive changes to derive an acceleration value during the time the stiction condition is present, which affects both device reliability and performance.
Some prior art approaches allow to improve stiction robustness of a MEMS device, for example, by increasing material stiffness and, thus, the mechanical restoring force in order to aid releasing the adhered parts of the device. Other approaches seek to improve surface conditions during the fabrication process in order to minimize stiction. However, such improvements result from design tradeoffs that come at the cost of reduced device performance, increased device size, and/or increased cost of manufacturing. What is needed are tools for MEMS designers to overcome the above-described limitations without increasing device size or sacrificing device performance.